What made Ubuntu different was that at the time it was initially released no one else was focusing on desktop integration. Upstream has gotten better at this in the last five years, as have (to a certain extent) other distributions, but never forget that it was Ubuntu that pioneered the effort. Much like the early KDE days when the attitude was "Of course Linux can be configured to do that," but the reality was that it was difficult or impossible, Ubuntu took practical and user-visible problems and addressed them at a distribution level.
It wasn't good enough that it be possible to configure your hardware to work, for Ubuntu people it had to be entirely automatic. It wasn't enough that something could be installed, it had to be installed by default or, at least, come with default settings tuned as if it were part of the base install.
Ubuntu's focus on integration--the whole experience from start to finish--and not just checking off the "Do we have package $foo?" checkbox made it different and, to a certain extent, still makes it different.
I am a long time Debian user and a Debian fan. I don't like a lot of things Ubuntu does and a lot of ways that it works, but the Desktop Linux effort will long be in Ubuntu's debt for showing us that a distribution isn't simply upstream pre-compiled and that there's a lot of work (sometimes little things) that can be done to produce an experience that 'feels' better.
Windows converts and random end users may not be able to express what it is that made Ubuntu more appealing--they'll just say it seemed easy to use, or friendly--and that's kind of the point: it's not a specific feature, necessarily, it's the whole attitude of the distribution that makes it work.